Solar Hot Water Versus PV

With the price of solar PV so low now the whole design philosophy of off the grid systems is changing. Some systems are even going in with a very small or even no generator and double the PV size.

Solar Hot Water Vs Solar PV.... Either method of getting energy out of the sun is fine. While the direct to water heating approach is more efficient at extracting the energy from the sun for a given area of roof space, solar PV (electricity) has come down so much in price that it is now less cost for the same energy return. The only benefit that solar hot water has is the smaller area it takes up on the roof. At my place I have a 30 bar evac tube (the best technology) solar hot water system and I have 2.8KW of PV. I am not completely off the grid but I have a GIS system with a small battery bank. On a sunny day by about 11am my batteries are usually full and the solar array switches about 50% into the hot water system. The result: If I run just solar my whole system (500L) sits at about 45 - 55 degrees which is raised from about 40 degrees after morning showers. Add the excess PV energy in and at 6PM as I write this my hot water is sitting at 71Deg - in the summer I often drain about 200L into the kids swimming pool (when there are not water restrictions!)

Your Off Grid Solar PV Array will be designed to get you through most of the Winter (with a little help from your generator). Winter is when the sunshine hours are at their shortest so in the summer, spring and autumn months you will generally have a lot more energy that is available from your PV panels but you usually can't use this because your household loads are the same and your batteries are fully charged well before lunchtime, so your solar array pretty much turns off during the long sunny afternoons. This energy can easily be put directly into electric water heating or other summer appliances like swimming pool pumps/heating and air conditioning (I know!, Air conditioning running off solar panels seems absurd.)

The next big question is this: Will you be using a fire with a wetback?


If Yes then this will see you through the winter for most of your hot water needs.

The last part of the solution is: What to do for a backup if you have a couple of weeks with no sun. - this applies if you have solar hot water too! The best solution to this is usually to have a Gas Instant Water heater in series with your hot water cylinder so that if the water in the cylinder gets below 40 degrees the power to the gas water heater switches on automatically. There are also other solutions that include extracting heat (or electric) energy from the generator.

As a general rule you are better off to put the money into extra solar PV panels rather than investing in two systems. I have 3 friends who install solar hot water and they have told me they would not install a hot water system into their place now - they would put it all into PV.

Here is a video discussing the differences.